In the manufacture of glass fibers it has been conventional to employ glass batch ingredients that contain boron and fluorine. These ingredients of glass batch act as fluxes during melting. Because the boron and fluorine constituents of a glass batch are volatilized during melting, varying quantities of boron and fluorine find their way from the furnace employed to the furnace stacks and ultimately the atmosphere. Since boron and fluorine are considered pollutants their emissions are controlled by environmental regulations which necessitates, in addition to careful control of furnace operations, the use of expensive and cumbersome pollution control equipment such as filters, scrubbers, electrostatic precipitators and the like, alone or in combination, to control their release to the atmosphere within the regulated values.
For these reasons, the art is constantly searching for glass batches which eliminate the use of boron or fluorine while still producing glass equivalent to or better in overall properties to the glasses made from glass batches using them.
Thus in U.S. Pat. No. 4,026,715 a silica, alumina and alkaline earth metal oxide glass composition is described which contains 2-4 percent by weight TiO.sub.2. In addition, this glass composition also contains zirconium oxide.
U.S. Pat. No. 3,876,481 also describes a glass composition which contains TiO.sub.2 in the range of 2-5 percent by weight as well as the silica, alumina and alkaline earth metal oxides normally employed in making a fiber glass composition. This composition also contains Li.sub.2 O in varying concentrations.
U.S. Pat. No. 3,847,627 similarly discloses a glass composition containing in addition to the silica, alumina and alkaline earth metal constituents normal in a fiber glass composition, 2 to 4 percent titanium dioxide and 1 to 5.5 percent of an oxide of zirconia.
U.S. Pat. No. 3,929,497 is another boron free, fluorine free glass composition suitable for manufacturing fibrous glasses which contains titanium dioxide in the range of 0.5 percent to 5 percent by weight and Fe.sub.2 O.sub.3 in quantities from 5 to 15 percent by weight.
Another glass composition known to the art that is boron free and fluorine free is described in U.S. Pat. No. 4,199,364. This composition contains Li.sub.2 O in the range of 0.1 to 1.5 percent by weight in addition to the silica, alumina and alkaline earth metal oxides normally employed and may also employ quantities of barium oxide.
In U.S. Pat. No. 3,095,311 another glass fiber composition is described which in addition to silica, alumina and alkaline earth metal oxide contains titanium dioxide and must contain certain claimed quantities of lithium oxide.
In U.S. Pat. No. 3,847,626, a glass composition suitable for manufacturing glass fibers is disclosed which contains in addition to SiO.sub.2, Al.sub.2 O.sub.3 and alkaline earth metal oxides, titanium dioxide in quantities ranging from 3-5 percent by weight.
Glasses which are boron and fluorine free and which contain titanium dioxide in the higher level ranges, i.e., 2 percent and above, tend to have liquidus temperatures considerably greater than those, for example, found in a conventional fiber glass batch such as "621" glass described in U.S. Pat. No. 2,571,074 and "E" glass.
In addition to the above patents, Applicant is aware of one commercial glass fiber that contains in addition to SiO.sub.2, Al.sub.2 O.sub.3, CaO, MgO and Na.sub.2 O, a low F.sub.2 content of 0.19 percent and TiO.sub.2 in an amount ranging from 1.8 to 2.25 percent. Applicant is also aware of another commercial glass fiber which contains TiO.sub.2 at a level of about 1.54 percent but which uses a boron level of about 0.47 percent and F.sub.2 at about 0.14 percent.
While all of the above described glasses are fluorine free and boron free or contain a substantially reduced boron and fluorine content over conventional "E" glass, they are not ideal glasses for fiber formation for various reasons. Frequently, these glasses tend to have a liquidus temperature which is close to a temperature corresponding to a glass viscosity of log 3 (the lowest practical fiber forming viscosity) which renders their use in a glass fiber forming environment less than desirable.
Further tests conducted on the two known commercial glasses mentioned above show that their electrical leakage value measured in accordance with the test described hereinafter render them unsuitable as products in electrical applications or as reinforcements for electrical resin laminates such as used in circuit boards.
The standard glass fibers used in industry today for electrical applications are typically formed from "E" glass the composition of which is described in U.S. Pat. No. 2,334,961 and broadly is comprised of 52 to 56 SiO.sub.2, 16 to 19 CaO, 3 to 6 MgO, 12 to 16 Al.sub.2 O.sub.3, and 5 to 12 percent B.sub.2 O.sub.3.
Another glass used for electrical applications is a modified "E" glass known as "621" glass and it is described in U.S. Pat. No. 2,571,074. This glass contains upwardly 52-56 percent SiO.sub.2, 12 to 16 percent Al.sub.2 O.sub.3, 19 to 25 percent CaO, and B.sub.2 O.sub.3 and fluorspar combined, at 8 to 13 percent with the fluorospar being present up to 3 percent.
By virtue of the instant invention, Applicant has now discovered that TiO.sub.2 containing glass fibers which are boron and fluorine free or which contain low boron and fluorine can be made such that they possess properties rendering them suitable as reinforcements in electrical applications. Applicant has found that by reducing seed counts in TiO.sub.2 containing glass fibers to certain levels, acceptable fibers can be made for these electrical applications and the fibers will have electrical properties nearly equivalent to those found in fibers made from conventional "E" and "621" glasses.
Further in the preferred embodiment of the invention the Applicant has discovered glass fibers having a composition which is characterized by having excellent liquidus properties and viscosity properties rendering it ideal for glass fiber forming while eliminating the need for boron and fluorine as batch ingredients. The fibers formed from the preferred embodiment were found to possess electrical properties such that the fibers can be readily used as a replacement for the industry standard glasses, such as "E" and "621" glasses in electrical applications and possess enhanced tensile strengths over these standard glasses.